1. Field of the Invention
The present invention generally relates to a balun device for performing unbalanced-to-balanced signal conversion, a balance filter device formed by integrating the balun device and a filter together, and a wireless communication apparatus integrating a balun device and a filter thereinto. More specifically, the invention relates to a balun device, a balance filter device, and a wireless communication apparatus, that can be effectively miniaturized.
2. Description of the Related Art
A typical wireless communication apparatus includes various radio frequency (RF) devices, such as an antenna, a filter, an RF switch, a power amplifier, an radio-frequency integrated circuit (RF-IC), and a balun device. Resonant devices, such as an antenna and a filter, handle unbalanced signals based on a ground potential, while a radio-frequency integrated circuit (RF-IC), which generates and processes radio frequency signals, handles balanced signals. Accordingly, a balun device, which serves as an unbalanced-to-balanced converter, is used for connecting the two types of devices.
Many balance filter devices formed by integrating a balun device and a filter together have been invented. Accordingly, the size of wireless communication apparatuses integrating such balance filter devices is becoming smaller. This type of balance filter device is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2003-087008. The balance filter device disclosed in this publication has a structure in which a filter designed by using ¼-wavelength resonators and a balun device are mounted on a dielectric substrate, and dielectric layers forming the filter and dielectric layers forming the balun device are laminated to integrate the filter and the balun device together.
The above-mentioned publication also discloses a structure in which a DC power layer is integrated into the balun device to allow an RF-IC to handle balanced signals superimposed on DC components, thereby achieving a further reduction in the balance filter device. In this publication, two structures concerning the arrangement of DC power layer have been proposed. In one structure, the DC power layer is disposed outside a ground (GND), as shown in FIGS. 15, and 25, and in the other structure, the DC power layer is disposed between the unbalanced terminal electrode of the balun device and the GND electrode.
However, in the structure in which the DC power layer is disposed outside the GND, due to a poor connectability between the DC power layer and the balanced resonance electrode, the provision of a DC-wiring through-hole for the GND electrode is required. Accordingly, the size of the balance filter device cannot be sufficiently reduced.
In the structure in which the DC power layer is disposed between the unbalanced terminal electrode and the GND electrode, the following problem in designing occurs. A large stray capacitance is generated between the unbalanced terminal electrode and the DC power layer so as to change the impedance. Accordingly, the impedance of the balun device when viewed from the filter is decreased, thereby making the matching of impedance difficult.